An automobile includes a number of systems using DC brush motors, for comfort and usability. For example, an air conditioner is provided with several DC motors for driving a door for changing its outlet and an air mix amount. The positions of door mirrors, sheet positions, and window positions in a power window system are operated by DC motors used as actuators. Furthermore, DC motors are also used in a system that changes the optical axis of headlight according to a steering turning angle.
In these systems, the positions of driving targets such as door positions, mirror positions, sheet positions, window positions, headlight positions are controlled by DC motors. Therefore, it is important to detect the positions of driving targets, that is, motor rotation information.
Conventional methods of detecting the positions of driving targets (motor rotation information) include a method of determining positions by measuring motor rotation information using Hall sensors and the like as described in JP 2003-049586A, and a method of determining positions by potentiometers. However, since such methods require sensors such as the Hall sensors and potentiometers, an increase in costs of the sensors and sensor mounting, and reduction in reliability due to spatial limitations, an increase in the number of signal lines, and sensor life cannot be ignored.
Accordingly, a method not requiring such sensors is proposed. According to the method, a ripple component waveform and a surge component waveform superimposed on a signal waveform (driving signal waveform) of one of voltage between the terminals of a motor and a current flowing through the motor are extracted, and motor rotation information is obtained based on the extracted ripple component waveform and surge component waveform.
Specifically, when a DC brush motor is driven, a driving signal waveform of the motor is represented as a surge component waveform due to discontinuity at the time of switching between the brushes and segments of commutators, added to a ripple component waveform of a specified cycle. For example, in the case of a motor having two brushes and three commutators, six ripples and surges occur in a driving signal waveform of the motor per rotation. Therefore, pulse signals corresponding to motor rotation are generated by extracting signals corresponding to a ripple component waveform and a surge component waveform from the driving signal waveform of the motor through appropriate filters and the like, and binarizing the extracted signals based on specified threshold values. By successively counting the number of pulses of the pulse signals thus generated, motor rotation information is obtained.
The surge component waveform is a high-frequency signal. Therefore, it is easily separated from low-frequency components attributable to fluctuations of external loads applied to a motor shaft and fluctuations of motor rotation speeds, contained in the driving signal waveform together with the surge component waveform. Therefore, methods of using a surge component waveform are considered to be particularly useful to detect motor rotation information, as proposed in JP 2000-308390A and JP 2001-138812A.
FIGS. 8 and 9 show examples of rotation information detection conditions of a device that obtains motor rotation information based on a surge component waveform. FIG. 8 is a time chart showing the transition of rotation speeds of a motor and the transition of a surge component superimposed on a motor voltage waveform between the terminals of the motor. FIGS. 9A and 9B are circuit diagrams showing the driving conditions of the motor in the operation modes of steady operation and braking operation.
As shown in FIG. 8, the motor is in steady operation until switching to braking operation at timing T11. As shown in FIG. 9A, an appropriate driving signal is fed to transistors constituting a bridge circuit, which is a driving circuit of a motor M, and a driving voltage Vd is applied to the motor M. When the motor M is switched to braking operation at timing T11, that is, as shown in FIG. 9B, when an appropriate driving signal is fed to the transistors of the bridge circuit and the terminals of the motor M are short-circuited, regenerative braking is applied to the motor M and its rotation speed begins to decrease.
Since a surge component waveform is attributable to self-induction of a motor coil when the segments of brushes and commutators are switched, when a current flowing through the motor (motor coil) is smaller, its signal strength becomes smaller. An induction current is fed to the motor M by counter-electromotive force due to inertia rotation (power generation operation) during braking operation. However, since the induction current decreases as the rotation speed of the motor M decreases, a surge detection error (counting error) E or the like might occur immediately before the motor stops (timing T12). Furthermore, such detection errors are accumulated in applications in which start and stop of the motor are frequently repeated, possibly leading to a large detection error.